A number of methods have been developed for the stereoselective synthesis of substituted alkenes (olefins). .beta.-Lactones (also known as 2-oxetanones) are known to decompose at moderate temperatures into alkenes and carbon dioxide. This thermal lability imparts to the oxetanone ring system a unique property for serving as an alkene precursor. However, the synthesis of alkenes from .beta.-lactones has not been widely utilized due to the lack of a practical and convenient method for synthesizing .beta.-lactones.
Classical methods for .beta.-lactone synthesis include internal nucleophilic displacement of halide ion from .beta.-halocarboxylate salts and the cycloaddition of aldehydes and ketones with ketenes. Alternative methods of .beta.-lactone synthesis include halolactonization of .beta.,.gamma.-unsaturated carboxylic acids, deaminative cyclization of .beta.-amino acids, the photocyclization of cinnamic acids, the photooxidation of 2-alkoxyoxetanes, and the palladium catalyzed carbonylation of halohydrins [H. E. Zaugg Organic Reactions, 8, 305-363 (1954); G. Searles in Comprehensive Heterocyclic Chemistry; A. R. Katritzky and C. W. Rees Eds.; Pergamon: Oxford, 1984; Vol. 7, Chapter 5.13]. W. Adam et al. describe a process for converting .beta.-hydroxy carboxylic acids to .beta.-lactones by treatment with benzenesulfonyl chloride in pyridine at subambient temperatures [W. Adam et al., J. Am. Chem. Soc. 94, 2000-2006 (1972)].
Wemple reported in 1975 [J. Wemple, Tetrahedron Lett. 3255-3258 (1975)] that lithium enolate derivatives of thiol esters add to aldehydes and ketones at -78.degree. C. to yield, after quenching at -78.degree. C., .beta.-hydroxy thiol esters. Masamune reported that .beta.-hydroxy thiol esters can be cyclized to form .beta.-lactones by treatment with mercury (II) methanesulfonate, which is an expensive and toxic reagent [S. Masamune et al., J. Am. Chem. Soc. 98, 7874-5 (1976)]. Two examples of the "spontaneous" cyclization of thiol ester-ketone adducts to form .beta.-lactones have been described [D. Seebach and R. Haner Chemistry Lett. 49-52 (1987) and J. Wemple et al. J. Org. Chem. 40, 3173-8 (1975)]. However, both of these cases involve unusual functionalized thiol esters, and in each case the .beta.-lactones were formed as minor by-products of the reaction of main interest.
.beta.-Lactones are important synthetic intermediates which can efficiently and conveniently be converted into other organic functional groups. In particular, .beta.-lactones are easily transformed into alkenes under mild conditions. The alkene functional group is incorporated in the structure of numerous compounds with commercial importance such as pharmaceuticals, agricultural chemicals, flavor and fragrance compounds, etc. The development of a more convenient and efficient method for the synthesis of alkenes greatly facilitates the preparation of compounds whose structures incorporate the alkene functional group.